A fuel assembly in a boiling water nuclear reactor consists of an elongated tubular container, often with a rectangular or square cross section. The container is open at both ends forming a continuous flow passage. The coolant of the reactor is able to flow through the passage. The fuel assembly comprises a large number of equally elongated tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. At the top, the fuel rods are retained by a top tie plate and at the bottom by a bottom tie plate. To allow coolant in the desired manner to flow past the fuel rods, it is important to keep them at a distance from each other and prevent them from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction.
A fuel assembly for a pressurized-water nuclear reactor has, in principle, the same design as a fuel assembly for a boiling water nuclear reactor, apart from the fact that the fuel rods are not enclosed by any tubular container and the fact that their number is higher.
Between the fuel rods secondary channels are formed, through which the coolant flows through the fuel assembly.
An important factor when designing a spacer for a light-water reactor is to ensure that the spacer brings about a good cooling of the fuel rods by a suitable mixing of the coolant. In those cases where the cooling is not sufficient, so-called dryout may occur in a boiling water reactor. A so-called DNB (Departure from Nucleate Boiling) may occur in a pressurized-water reactor. In serious cases, dryout and DNB, respectively, give rise to penetration of the fuel rods.
The spacers thus influence the flow of the coolant and hence the cooling of the fuel. It is known that, in a region immediately below the spacer where the coolant has still not passed through the spacer, a deterioration of the coolant film on the fuel rods occurs. On the other hand, in a region above the spacer, where the coolant has just passed through the spacer, a reinforcement of the water film instead occurs. The reinforcement of the coolant film is due to the turbulence which arises in the coolant when it passes through a spacer. The greatest risk of dryout exists in the upper part of the fuel immediately below the spacers.
One way of increasing the cooling of fuel rods by means of the arrangement of guide vanes at the downstream edge of the spacer in the fuel assembly is described in PCT patent document WO 91/13442. Two or four guide vanes are arranged extending from the mentioned edge and curved in such a way that a swirl is formed in the upwardly flowing coolant. Further, the guide vanes are exposed and may become damaged in connection with the mounting of the fuel rods.
Another method is described in Swedish patent document PCT patent document SE 9303583-0. The spacer described in this document comprises an orthogonal latticework formed from sleeves. In this spacer, guide vanes are formed as tabs in the sleeve material. The disadvantage of this solution is that the larger the tab chosen, the larger part of the supporting length of the fuel rod is lost.
Known spacers are often designed completely or partially of Inconel which is a material with a good strength and good properties as far as manufacture of spacers is concerned. However, it is desirable to manufacture spacers of a material with less ability to absorb neutrons, that is, a low-absorbing material, for example a zirconium alloy. Spacers of zirconium alloy are an advantage from the point of view of reactivity economy. In addition, spacers of a low-absorbing material are less radioactive after irradiation. This is an advantage when handling these spacers after reactor operation.
The object of the invention is to provide a spacer which gives good cooling, small flow resistance, and which is simple to manufacture and, in addition, is possible to construct of zirconium alloy.